1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include one or more base stations, which may also be referred to as node-Bs or Access Network (AN), for providing wireless connectivity to one or more mobile units, which may also be referred to using terms such as user equipment, subscriber equipment, and Access Terminals (AT). Exemplary mobile units include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop computers, desktop computers, and the like. Each base station may provide wireless connectivity to one or more mobile units, such as the mobile units in a geographical area, or cell, associated with the base station. Each mobile unit may also maintain one or more wireless communication links, or legs, with one or more base stations. For example, a mobile unit can maintain a plurality of wireless communication links with an associated plurality of base stations. A mobile unit that is in communication with more than one base station over more than one leg is conventionally referred to as a mobile unit in “soft handoff” with the plurality of base stations.
Resources for communication between the base stations and the mobile units over the wireless communication links may be allocated using bits that are transmitted over channels of the wireless communication links. For example, power control bits may be transmitted from a base station to one or more mobile units over a shared channel. The power control bits may be used by the mobile units to adjust reverse link transmission powers used to transmit information from the mobile units to the base station. The base station transmits each power control bit at a power level that may be selected based upon a desired bit error rate or frame error rate associated with the transmitted power control bit. However, the overall power budget for transmitting power control bits may be limited to a maximum total power. In some cases, the sum of the power levels selected to transmit the power control bits may exceed the maximum total power budget. Conventional base stations may therefore scale down, or clip, the power levels for the power control bits so that the sum of the power levels is less than the maximum total power budget.
Scaling down the power levels used to transmit the power control bits may increase the number of error bursts on the reverse link and, consequently, may degrade the quality of the wireless communication link. For example, when a mobile unit is in soft handoff with a large number of base stations in an active set, many of the legs may be relatively weak, which may increase the probability that the base stations may transmit power control bits including UP commands over the associated legs. The error rate for the power control bits transmitted over the relatively weak legs may be larger than the error rate for relatively strong legs, so the probability that the mobile unit may incorrectly decode an UP command as a DOWN command when the associated power control bit is sent over a relatively weak leg may also be larger. A single incorrectly decoded UP command may cause the mobile unit to erroneously decrease its reverse link transmission power. For example, the mobile unit typically applies a voting rule to adjust the reverse link transmission power. In one exemplary voting rule, the mobile unit will adjust its reverse link transmission power downward if it receives at least one DOWN command in a power control bit received over one of the legs in the active set.
Reducing a reverse link transmission power based on an incorrectly decoded power control bit (e.g., interpreting an UP command as a DOWN command) may lead to one reverse link packet error or a burst of reverse link packet errors. A burst of reverse link packet errors may degrade the overall performance of the wireless communication link. Frequent bursts of errors experienced by some mobile units may lead to reduction of overall reverse link radiofrequency capacity. For example, outer loop power control systems implemented in base stations typically respond to reverse link errors by increasing a set point, which may be used to determine whether to transmit an UP command or a DOWN command in each power control bit. Conventional techniques increase the set point by a relatively large step size when a frame is successfully decoded and decrease the set point by a relatively small step size when a frame is unsuccessfully decoded. Accordingly, a burst of errors can increase the set point relatively rapidly, whereas the set point may not return to a nominal target value until a relatively large number of frames have been decoded successfully. The high set point of one of the mobile stations determined by a base station may lead to an increase of reverse link transmission powers for all the mobile units that are in communication with the base station because they may need to overcome the interference from the mobile station with the high set point. Consequently, an erroneously high set point may degrade the overall performance and capacity of the wireless communication system.